This invention relates to an improved sound attenuating device in particular to an improved muffler system for fluid flows.
Present attenuators and muffler systems utilize baffle systems, retroverted systems or expansion chambers with the sound pressure level of the fluid flow through the muffler system generally decreasing continually from the inlet of the muffler to the outlet of the muffler. Because of the particular structures of the prior art attenuators and muffler systems, these structures have fixed static flow resistance to the fluid flowing therethrough.
According to sound attenuating theory, maximum sound attenuation occurs when the sound pressure level is matched with the flow resistance of the sound absorbing features of the attenuator. As the sound pressure level in a gas flow decreases as it proceeds through prior art attenuators having a fixed flow resistance, the decrease in sound pressure level of the gas flow is matched at only discrete levels of static flow resistance.
There is no suggestion in the prior art that an interaction exists between the sound pressure level of the gas flow to be attenuated and the changes in the static and dynamic flow resistance, such that for decreasing sound pressure levels, the dynamic flow resistance should be maintained at an optimum for the system, by increasing the static flow resistance. Accordingly, an attenuating device having a changing flow resistance to continually match the decrease in sound pressure level is not known in the prior art.
The present invention is directed in particular to this problem and describes a sound and noise attenuating device for use with flowing gas systems, wherein the flow resistance is constantly varied to match the decrease in sound pressure level in order to obtain maximum attenuation. In particular the invention is directed to a free flow sound attenuating device wherein the sound attenuated encompasses sound in a flowing gas having a decreasing sound pressure level as it passes through the attenuator. The sound absorbing features of the attenuator of the present invention are structured so that the flow resistance changes to match the decreasing sound pressure level of the flowing gases. The invention is particularly useful, but not limited to, flowing gases of single phase. By single phase it is meant flow comprising substantially 100 percent gaseous state with little or none of the flow in a liquid state. The device may be used for intake and exhaust gas flow systems such as mufflers and resonators for internal combustion engines.
It must be noted that this invention is particularly concerned with the change in sound pressure level and not sound power. Sound power is a measure of the total sound radiating from a device whereas sound pressure level is the strength of a sound wave after it travels a specified distance from the sound source. Therefore, if the sound pressure level can be controlled, the amount of decrease in sound power is a function of the efficiency of an attenuator, with a desired decrease in sound power being easily corrected by increasing the size and capacity of the attenuator.